There are various prospects for linear α-olefins, notably those containing 4 to 20 carbon atoms, depending on the length of their carbon chain. For example, C4 to C8 olefins are mainly used as co-monomers for the production of low-density polyethylenes (LLDPE), C8 to C14 olefins as intermediates in the lubricant industry, and C10-C18 olefins for the production of detergents. These olefins experience a strong economic growth. Most industrial α-olefin production processes are ethylene oligomerization methods catalyzed by transition metal complexes (Ni, Ti, Zr) or AlEt3 (Alpha Olefins Applications Handbook, G. R. Lapin and J. D. Sauer Eds M. Dekker, NY, 1989). Most of these processes lead to Schulz-Flory type distributions, which can be quantified by value K. This value K represents the chain propagation probability and it is experimentally determined by the Cn+2/Cn molar ratio. The values of K that are encountered in industrial processes are of the order of 0.7-0.8, which corresponds to wide α-olefin distributions ranging from C4 to C20+. It is then difficult to upgrade all of the products formed, in particular the “waxes” (olefins>C30).
On the other hand, not insignificant amounts of branched or internal olefins can also be produced by such methods, olefins that are often difficult to separate and unwanted for industrial processes. It is therefore important to develop new methods allowing these olefins to be minimized.
Incessant research work has been done during the past years in order to find new catalytic systems allowing to obtain narrower olefin distributions, for example allowing to optimize the formation of C4-C10, more selective in linear alpha-olefins and more active.
Relatively recently, catalytic systems comprising group 8-10 transition metals such as iron, nickel, palladium and cobalt associated with diimine type chelate ligands have been developed and applied for ethylene polymerization or copolymerization of alpha-olefins or olefins carrying a function, such as methyl acrylate (see for example the review by V. Gibson in Angew. Chem. Int. Ed. 1999, 38, 429). These systems most often use a co-catalyst that is an aluminium derivative such as an aluminoxane.
More recently, it has been shown (S. Svejda et al., Organometallics, 1999, 18, 65-74; WO-A-96/2310; WO-A-00/10,945; and U.S. Pat. No. 5,880,323) that systems comprising a nickel complex associated with an α-diimine type ligand in the presence of a Lewis acid or a Bronsted acid, more particularly an aluminium derivative such as an aluminoxane or an alkylaluminium chloride, allow to catalyze the oligomerization of ethylene to linear α-olefins. However, in these systems, the amounts of aluminoxane used are generally high (more than 100 equivalents per mole of nickel) and the distribution of the olefins formed is wide: from C4 to C20, with a Schulz-Flory constant generally above 0.6.
It has also been shown that some iron complexes associated with bis(imino)pyridine type trident chelate ligands, activated by an aluminium alkyl derivative and more particularly an aluminoxane, catalyze the oligomerization of ethylene (see the review by V. Gibson, Chem. Rev. 2007, 107, 1745; Du Pont de Nemours WO99/02472; WO 02/06192 A1; Britovek et al, Chem. Eur. J., 2000, 6, 12, pp 2221-2231, BP Chemicals WO 99112981; Chevron Phillips WO2005/080301A 1).
The distribution of the oligomers formed essentially depends on the nature of the bis(imino)pyridine ligand, in particular the substituents on the aromatic rings of the imines. These distribution generally follow a Schulz-Flory type law whose characteristic factor K ranges from 0.70 to 0.85 (M. Brookhart, S. Brooke, J. Am. Chem. Soc. 1998, 120, 7143-7144).
Shell International Research (WO 01/58874 A1; WO 02/00339 A2, WO 2004/037415 A2; WO 2007/059015) teaches that the use of dissymmetric pyridine bis-arylimine ligands associated with iron complexes activated by an aluminoxane allows to obtain Schulz-Flory linear olefin distributions (no deviation) with a high minimization of the production of mass products.
Exxon Mobil describes, in patent application US-2005/0,192,470, a method of producing linear α-olefins whose chain length does not exceed 12 carbon atoms (short Schulz-Flory distribution, K=0.45) when using the 2,6-bis-phenylimine-pyridine ligand associated with a complex, preferably a Fe(II) complex, and an aluminoxane.
These complexes, which carry very weakly encumbered bis-aryliminie-pyridine ligands, lead to the shortest oligomer distributions. However, they involve the drawback of being poorly stable and they are rapidly deactivated, particularly with temperature.
Surprisingly enough, we have discovered that when the complexes resulting from the combination of a nitrogen-containing ligand obtained by reaction of a compound X with a compound Y, whose general formulas are described hereafter, with an iron compound are previously subjected to an oxidation stage, novel precursors of olefin oligomerization, co-dimerization or polymerization catalysts that do not involve the drawbacks of the aforementioned systems are obtained.